Throughout the last 15 to 20 years, earth leakage circuit breakers (ELCBs) have been used with increasing frequency to protect against earth leakage faults. The principle of the ELCB, as is well known, is that in the event of an earth fault, some of the current which normally flows through the live feeder and returns through the neutral, leaks to earth. Consequently, there exists an imbalance between the respective live (or phase) and neutral currents. The imbalance is detected and, if it exceeds a predetermined threshold level, a relay is energized which interrupts the electrical supply voltage fed to the live and neutral feeders.
The value of ELCBs is particularly manifest when protecting against earth leakage in an appliance having an electrically conductive housing which is connected to ground. In such case, if a fault occurs in the appliance whereby the casing becomes live, then the earth leakage current flows to ground and immediately operates the ELCB. However, if the ground connection is faulty such that there is no leakage path for current to flow in the event that the casing becomes live, then the ELCB will not become energized until somebody touches the electrically conductive housing and thereby provides a leakage path to ground. In such case, the leakage current passes through the person giving rise to the required imbalance between the live and neutral feeder currents which causes the ELCB to operate. Under these circumstances, whilst the ELCB will still operate, there is an inevitable earth leakage through the person who touches the appliance.
Thus, the ELCB must be so adjusted that the leakage current which occurs in such circumstances is not fatal. However, what constitutes a "fatal" leakage current varies from one person to another. Moreover, the leakage current which will actually flow through a person consequent to the casing of an electrical appliance becoming "live" is a function of the person's body resistance. People whose skin is apt to be moist (mothers and children, for example) have a much lower skin resistance than building site personnel, for example, whose skin tends to be dry and callose. A leakage current which might give the latter a slight shock might nevertheless prove fatal for the former. Furthermore, the magnitude of leakage current which could be fatal for a youngster is so low that to adjust the ELCB so as to be effective against such a small leakage current would, in practice, result in false alarms and unnecessary tripping of the ELCB.
There is, in fact, no perfect solution to this problem. The best that can be done is to adjust the ELCB to operate at an "average" leakage current, typically in the order of 30 mA, mindful of the fact that such a current can indeed be fatal for a segment of the population. When it is further borne in mind that, if the ground connection of an appliance is absent or faulty, then a leakage current will generally find a return path to ground through a person who inadvertently touches an appliance's metal casing, it is apparent that, in some cases at least, ELCBs give an apparent, but not altogether justified, impression of safety.